Formula Fields and Calculated Measures
Unlike formula fields, which are created at the dataset level and evaluate results row by row, calculated measures are made at the workbook level using aggregated results. For example, if you create a formula field for sales amounts, each row in the dataset will show one value based on your formula. If you then use that formula field as a measure in a pivot table or chart, the results are simply an aggregate of the formula field values. In contrast, calculated measures use the aggregated results of base measures and arithmetic operations to show things like year-over-year variances and profit percentages. You can't get these kinds of results using formula fields alone.
The following examples show why you get a different result in each case:
For more information about formula fields, see Formula Fields.
For more information about calculated measures, see Calculated Measures.